UCB: Specify the VTOC to Be Tested

UCB=(reg): Supplies the address of the UCB for the volume whose VTOC is to be tested. The UCB address can be for a captured UCB, or for an actual UCB above or below the 16 MB line. If your program is in 31-bit mode, this address must be in 31-bit address; the high order byte is part of the address.
Recommendation: Code the address of the UCB parameter as register (2-12). Coding an RX-type address gives unpredictable results.

The CVAFTST macro accepts the address of a UCB or UCB copy. Unauthorized programs can get a copy of the UCB by using the UCBSCAN macro and specifying the COPY, UCBAREA, CMXTAREA, and DCEAREA keywords. The UCB copy and common extension copy must be below the 16 MB line and on a word boundary. Data accessed with DCEAREA can be above the 16 MB line. Refer to z/OS HCD Planning z/OS HCD Planning for details.